Evolution of branching filamentous cyanobacteria: Molecular-phylogenetic analy- ses of stigonematalean species

Modern cyanobacteria form a morphologically diversified group. In traditional taxonomy, morphological characters are used to divide them into five subsections [Rippka et al., 1979; Castenholz, 2001]. Cyanobacteria of subsection I (formerly order Chroococcales) and II (Pleurocapsales) are unicellular coccoids. Those of subsections III to V have filamentous forms with varying morphological complexity. Filaments of subsection III (Oscillatoriales) have only vegetative cells, whereas in subsections IV (Nostocales) and V (Stigonematales) vegetative cells can differentiate into heterocysts or akinetes depending on growth conditions. Heterocystous species are divided into two subgroups, subsections IV and V. Subsection IV consists of heterocystous cyanobacteria whose cells always divide along with the longitudinal axis of a trichome and therefore filaments are uniseriate and one-dimensional. In cyanobacteria of subsection V, some cells in a trichome have additional division plane(s) and their filaments exhibit true-branchings in all members and are multiceriate in some cases. Cyanobacteria of subsection V present the highest degree of developmental complexity in prokaryotes, characterized by cell differentiation and true-branching filaments. Elucidation of how cyanobacterial complex morphology evolved is a key to an understanding not only of their evolutionary history but also of prokaryotic developmental systems. Many Precambrian microfossils have been linked to modern cyanobacteria by comparative study between fossil and extant taxa [Knoll and Golubic, 1992]. However, geological records are always accompanied by incomplete preservation and the evolutionary history of cyanobacteria has remained unclear. In the past few decades, spectacular progress has occurred in molecular biology, and molecular phylogeny has become a powerful tool in elucidating biological evolution, owing to tremendous accumulation of sequence data and splendit progress in computational analyses. Molecular data have been used to reconstruct cyanobacterial phylogeny so far. Most are based on 16S rRNA sequences [e.g., Giovannoni et al., 1988; Turner et al., 1999; Wilmotte et al., 2001; Gugger et al., 2004]. Those studies suggested monophyly of heterocystous clade (subsection IV and V) and that of cyanobacteria bearing true branching (V). Some phylogenetic trees are constructed based on other gene sequences, such as nifH [Zehr et al., 1997] or nifD [Henson et al., 2004]. However, the nifH and nifD analyses do not support monophyletic origin of stigonematalean taxa. Moreover, most of the phylogenies presented to date contain only two genera (Chlorogloeopsis, Fischerella) of subsection V, due to lack of axenic cultures, and could not resolve the evolutionary history of branching filamentous cyanobacteria. Here the author carried out a molecular phylogenetic study of stigonematalean cyanobacteria to examine how morphologically complex cyanobacteria evolved. Seven strains distributed five genera of subsection V were chosen to represent morphological variety of the group. Multiple genes (16S rRNA, hetR) were sequenced and analyzed to clarify phylogeny.